A serious problem encountered in the operation of photoconductive infrared detectors is that of "sweep-out". This is the transfer of minority carriers from an active material to a contact, due to an applied electric field, in a time which is substantially less than the normal lifetime of these carriers. This reduced lifetime of the carriers degrades the effectiveness of the detector. Sweep-out is a chronic problem with most P-type detector materials with high ambipolar mobility and is becoming a greater problem with high performance N-type detector materials. It has been recognized that the problem of sweep-out can be reduced by the use of an isotype heterojunction between the active material and the contact to block the transfer of the minority carriers. However, there are serious problems in the design and fabrication of isotype heterojunctions to produce effective impedance-matched photoconductive infrared detectors.
In an impedance-matched infrared photoconductive (IMPC) detector the thickness of the active absorbing material is significantly less than the absorption depth, i.e., the depth over which the incident radiation is absorbed. To maintain high quantum efficiency, an impedance-matching substructure which normally consists of a dielectric multilayer spacer region and a high reflectivity groundplane is required. The active absorbing layer together with the impedance-matching substructure make up an optical cavity which traps the incident radiation allowing it to be completely absorbed in the active layer. In this way the quantum efficiency of an IMPC detector can be made comparable to a conventional photoconductor in which the thickness of the active material is comparable to the absorption depth of the incident radiation. A reduction in active material volume while maintaining high quantum efficiency leads to improved signal-to-noise. Examples of active layer alternatives include, but are not limited to, epilayer mercury cadmium telluride, epilayer indium antimonide, epilayer indium antimonide arsenide, epilayer gallium indium antimonide, superlattice indium antimonide/indium antimonide arsenide and superlattice indium arsenide/gallium indium antimonide.
In an isotype heterojunction blocking contact, a wider bandgap isotype semiconductor layer is grown epitaxially on the active layer with its characteristic bandgap energy. An example of a heterostructure blocking contact in a long wavelength infrared (LWIR) mercury cadmium telluride conventional photoconductive detector is described in "High Responsivity HgCdTe Heterojunction Photoconductor" by D. K. Arch, R. A. Wood and D. L. Smith in Journal of Applied Physics, Vol. 58 at pages 2360-2370 (1985).
Precise control of layer thickness is important for optimum performance of an impedance-matched photoconductive infrared detector. The present invention provides a structure which allows precise control of the layer thickness in a IMPC with the inclusion of an isotype heterojunction blocking contact and a method of fabrication for such a detector.